The cytoplasmic site of gene expression and use of virally encoded enzymes are distinguishing features of vaccinia virus and other poxvirus vector systems that probably accounts for their consistent ability to express foreign genes derived from a variety of prokaryotic, eukaryotic, and viral sources. This feature, together with their ability to stably integrate and package large amounts of additional DNA without loss of infectivity, their wide host range, and the development of methods for isolating recombinant viruses, account for their diverse use and popularity. The formation and isolation of recombinant vaccinia viruses remains a time consuming process especially when several genes need to be introduced into the vaccinia virus genome. We developed two simple new ways of selecting and screening recombinant vaccinia viruses that are dependent on plaque formation and expression of beta-glucuronidase, respectively. A new highly inducible recombinant vaccinia virus expression system that employs the Escherichia coli lac repressor and bacteriophage T7 RNA polymerase was developed. We have continued to explore the use of the highly attenuated and host restricted MVA strain of vaccinia virus as an expression vector because of the added safety. A recombinant MVA that expresses the T7 RNA polymerase gene was constructed for use as a transient expression vector in the laboratory. In addition, we have further evaluated the immunogenicity of a recombinant MVA that expresses the influenza virus hemagglutinin and nucleoprotein genes in a mouse model system.